1. Field of the Invention
The present invention generally relates to an automatic control system for controlling a chemical reaction in order to maximize efficiency, and more particularly to an automatic control system for controlling the operation of the attack tank and filtration unit in a phosphoric acid recovery plant in order to maximize the recovery of phosphoric acid.
Phosphoric acid (H.sub.3 PO.sub.4) is an important intermediate chemical product. It is used primarily by the fertilizer industry, but also is useful in a number of other areas such as in detergents, water treatment, and food products.
Phosphoric acid is primarily produced by what is known in the art as the "wet process." Using the wet process, phosphate rock, which comprises calcium phosphate [Ca.sub.3 PO.sub.4 ] and a number of impurities, is mined, mixed with water and then ground. Typically, ball mills or rod mills are used to obtain the desired size distribution. The ground wet rock then is fed to a reactor, or `attack tank`, and reacted with sulfuric acid. The process is represented by the following equation: EQU Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 +6H.sub.2 O=2H.sub.3 PO.sub.4 +3Ca(SO.sub.4)*2H.sub.2 O A
The phosphoric acid thus produced remains in solution; calcium sulfate, commonly referred to as gypsum, is a crystalline solid under reaction conditions.
Although phosphoric acid is represented as H.sub.3 PO.sub.4, plant output is often measured in tons of P.sub.2 O.sub.5, or phosphorous pentoxide. Approximately 3 tons of 30% phosphoric acid aqueous solution is equivalent to about 1 ton of P.sub.2 O.sub.5. It also is common in the trade to use the terms "phosphorous pentoxide" and "phosphoric acid" interchangeably. Thus, throughout the specification, use of the term "phosacid", when describing conditions in the attack tank, will refer to phosphorous pentoxide, and use of the term "phosacid", when describing the product from the filtration unit, will refer to phosphoric acid.
The reactor effluent, which comprises the phosphoric acid and gypsum, then is sent to a filtration unit wherein the solid gypsum is separated from the solution. The gypsum forms a filter cake and the filtrate comprises phosphoric acid solution. The filtrate is further treated before shipment.
The filter cake is washed with water to recover phosphorous values. Typically, a two or three-stage counter-current washing is utilized. Process water, which contains essentially no phosphorous values, is used to wash the gypsum cake a final time before the cake is removed as a by-product. The wash water, which contains some phosphoric acid, then is used in the penultimate washing, from which additional phosacid is recovered. In a third stage, the gypsum cake just separated from product acid is washed with the solution from the second washing. The solution from this third washing is introduced to the attack tank. This stream typically is known as an `acid-recycle stream`, as `return acid`, or simply as `return`.
Reactor and filtration operating parameters should be closely controlled to ensure maximum recovery of phosphoric acid. Maximum recovery of phosphoric acid does not necessarily correlate with maximum concentration of phosacid in the reactor or the maximum concentration of phosacid in the product filtrate stream. Rather, the maximum recovery of product is obtained by maintaining an optimum concentration of phosacid in the reactor.
As set forth in Equation A above, water and sulfuric acid are fed to an attack tank along with phosphate ore. Typically, the ore is fed in the form of an aqueous slurry, and acid solution is recycled from the cake washings. It is typical to add a stoichiometric excess of water.
Water may be added by way of various process streams, including the feed phosphate ore stream, the acid recycle stream from the filter, or a raw water feed stream. Water required to provide the stoichiometric quantity of water in accordance with the reaction set forth in Equation A may be added by way of any one of these streams. It is common in the trade to add the water to the attack tank by way of the return acid from the filter unit. However, an operator must take care not to add water to the filter in a quantity that will oversaturate the gypsum filter cake. As skilled practitioners recognize, it is not desirable for the moisture content of the filter cake to be excessively high because water remaining in the cake effects the water balance in the attack tank. It is desirable to maintain an overall water balance on the attack tank and filter to maintain the stoichiometric relationship set forth in Equation A in the attack tank while at the same time maintaining the efficiency of the filter unit and the moisture content of the filter cake.
Typically, in manual operation of a phosacid attack tank and filter, an operator would vary the amount of water added to the filter by visual inspection of the moisture content of the filter cake discharged and the knowledge of concentration of phosacid in the attack tank. For example, low phosacid concentration decreases cake discharge moisture content. As a consequence of the low phosacid concentration, the operator would decrease the amount of water added to the filtration unit. This decreases the rate at which water is added the filter, and thus, less water will be added to the attack tank by way of the return acid. The lower quantity of return acid causes the phosacid concentration in the attack tank to increase. As the phosacid concentration in the attack tank increases, the operator would add water to the filter in an attempt to bring the concentration of phosacid in the attack tank to its optimum value. However, this additional water would cause the moisture content of the gypsum filter cake to increase, and still more water would have to be added to the filter to rinse the filter cake. However, under these conditions, the ability of the filter to accept more water would be marginal at best, because the gypsum crystals would be affected by the acid concentration. Also, as the concentration of phosacid in the attack tank increases, the viscosity of the reaction effluent increases, and the phosacid tends to adhere more easily to the gypsum crystals. The additional acid content of the slurry leaving the attack tank therefore causes the filterability of the gypsum filter cake to decrease and further reduces the efficiency of the filter. Thus, production rates of phosacid would decrease even though the concentration of phosacid in the attack tank was high.
Thus, keeping the phosacid concentration in the attack tank essentially constant is desirable to maximize phosphoric acid yield. Because the concentration of the phosphate ore fed to the attack tank is constantly changing, maintenance of the concentration of phosacid in the attack tank is difficult. Also, the sulphate and phosacid concentrations in the attack tank influence the growth and type of calcium sulfate crystals formed in the attack tank. Skilled practitioners recognize that gypsum crystal size and characteristics directly effect the filterability and washability of the reaction slurry solid effluent from the attack tank. Thus, there is a need to control the operating conditions in the attack tank and filtration unit to ensure a maximum yield of phosphoric acid.
Many factors affect the ability to maintain the phosacid concentration in the attack tank at this optimum concentration. The concentration of phosphate ore in the rock fed to the attack tank constantly changes because, inter alia, the quality and fineness (which affects reaction rate) of the rock, and the type and quantity of impurities in the rock, vary widely among different ore sites, and even at the same site. The quality or grade of the rock is determined by the concentration of phosphate ore present in the rock and the type and concentration of impurities therein. The fineness of the rock is determined by the rock itself and the process and equipment used to crush the rock.
To maintain the concentration of phosacid in the attack tank at its optimum value, the amount of water added to the attack tank must be adjusted to maintain the stoichiometry of Equation A above. The water addition rate must also be adjusted to compensate for differences in reaction rate caused by differences in fineness of the rock. Failure to consider fineness and its effect on reaction rate may cause the concentration of phosacid in the attack tank solution to change even though the mass flow rate of feed rock is constant.
Filterability of the reaction slurry (attack tank effluent) also affects product recovery. Because the amount of phosphate fed to the attack tank is not constant, the amount of sulfuric acid needed to effect the reaction set forth in Equation A above also changes. The amount of product and by-product produced thus are constantly changing, and these changes affect the chemical makeup of the filter cake. Thus, rinse water may be required at the filtration unit to further rinse the filter cake and remove the phosacid entrained therein, but the additional water may not be necessary at the attack tank. Further difficulties arise when unexpected mechanical problems (i.e., broken pumps, stuck valves, and the like) affect the flow rates of attack tank feed and effluent streams, and may cause the phosacid concentration in the attack tank to worsen drastically before these problems are detected.
2. Description Of The Prior Art
A number of prior art systems control the production of phosphoric acid by focusing on the excess sulfate concentration in the attack tank. Historically, sulfate control was achieved by using large tanks for the attack process, periodically measuring the excess sulfate level in the tank, and making adjustments in the acid or rock feed rates accordingly. Other approaches utilize continuous analysis of the rock or continuous analysis of the sulfate in the attack tank. However, these measurement devices were prone to breakage, and usually required a full time engineer to maintain.
One such control system for a phosphoric acid recovery plant is disclosed in U.S. Pat. No. 4,777,027, directed to a method for preparing phosacid and calcium sulfate. In accordance with the method disclosed, flow rates of mixtures circulating about the attack tank (i.e., recycle streams) are adjusted relative to the base flow rate of the feed streams. Based on specified conditions, the system of this patent controls reaction conditions in succeeding reaction zones of a multistage attack tank system so that the concentration of calcium sulphate is maintained at a desired value. By maintaining the desired concentration of calcium sulphate, a desired concentration of phosphoric acid is produced.